Genomic imprinting is a molecular process that distinguishes the parental origins of certain genes, such that one allele is expressed and the opposite allele is silent. Many imprinted genes are expressed in the placenta, where some are known to play critical roles in placental development and metabolism. Most imprinted genes are organized in tightly linked clusters. There are at least 16 imprinted clusters in the mammalian genome, with the imprinting of each governed by a differentially methylated domain (DMD). One parental allele of a DMD becomes methylated duhng gametogenesis, and this methylation is perpetuated after fertilization by the action of the DNA cytosine methyltransferase 1 (DNMT1) enzyme. At the 8-celt embryonic stage this activity is from the maternal-effect isoform DNMTIo. Mouse embryos genetically engineered to lack oocyte-derived DNMTIo have 50% of the normal level of DMD methylation and are epigenetic mosaics of cells lacking DMD methylation on different imprinted clusters. Imprinting in the placenta is also severely disrupted in the absence of DNMTIo. Thus, the genetic model of DNMTIo deficiency is useful to study the role of genomic imprinting in placental function. The main objective of this research is to determine the global role of imprinting in the metabolic function of the mouse placenta by identifying the functional defects in DNMTIo-deficient placentas. This objective will be pursued in three Aims. Aim 1 : Define causal relationships between defects in imprinting and fuel metabolism in DNMTIo deficient placentas. The relationship among abnormalities in imprinted genes, morphology and metabolism in E9.5-E17.5 DNMTIo-deficient placentas will be determined using a combination of RNA in situ hybridization, histology and quantitative measurements of imprinted-gene expression and DNA methylation. We expect to find significant changes in the methylation of DMDs from one or more imprinted clusters and correlate these changes with specific morphological and metabolic defects. Aim 2: Identify novel pathways relevant to placental metabolism and function. The goal here is to use a variety of experimental techniques, including genome-wide expression studies, to identify established and novel metabolic pathways that are disrupted in DNMTIo-deficient placentas. We expect to find defects in the development of the maternal-fetal interface, lipid metabolism, and glycogen synthesis, breakdown and migration of glycogen producing cells. Aim 3: Determine the role of discrete imprinted genes in processing of placental metabolic fuels. Based on a number of criteria, the roles of individual imprinted genes in placental metabolism will be studied in knockout mice and transgenic lines overexpressing a gene to twice its normal level.